This invention relates to reduced density foam articles and methods of making same.
Methods for producing reduced density thermoplastic foam articles often employ flammable hydrocarbon gases as a physical blowing agent or employ complicated and costly crosslinking technologies. Nonvolatile gases such as carbon dioxide (CO2) and nitrogen are generally not preferred as a blowing agent for thermoplastics due to their low solubility in the polymer.
The present invention provides reduced density foam articles These articles can be made with environmentally-friendly gases.
In one aspect of the present invention, an unfoamed barrier layer on a foam layer is employed to produce reduced density foam articles. It is believed that incorporating an unfoamed layer onto a major surface of a layer of foamable material acts as a barrier and dramatically changes the diffusional characteristics of a fugitive gas in the foamable material. This provides the ability to foam materials that may otherwise be difficult to foam.
One aspect of the invention provides a method of making a multilayer reduced density foamed article comprising:
(1) mixing at least one thermoplastic polymer and at least one blowing agent that is, or that produces, a fugitive gas to form a foamable melt mixture,
(2) shaping the melt mixture such that it has at least one major surface,
(3) affixing or creating a barrier layer of nonfoaming material on one or more of said major surfaces, wherein the barrier layer inhibits diffusion of the fugitive gas out of the foamable melt mixture, and
(4) causing the melt mixture to foam.
Another aspect of the invention provides a foam article comprising a foam layer with a barrier layer on at least one major surface of the foam layer wherein the foam layer of the article has a lower density than a foam layer of a similar article without a barrier layer.
Another aspect of the invention provides a foam article comprising a foam layer with a barrier layer on at least one major surface wherein the article has a density lower than that of a foam article without a barrier layer.
Another aspect of the invention provides a foam article comprising at least four layers wherein the article comprises at least one foam layer between two unfoamed layers and wherein the article has a density lower than a similar article without unfoamed layers.
Another aspect of the invention provides a method of producing a reduced density foam article comprising:
1) mixing at least one thermoplastic polymer and at least one blowing agent that is, or that produces, a fugitive gas to form a foamable mixture,
2) shaping the melt mixture such that it has at least one major surface,
3) affixing or creating a barrier layer of nonfoaming material on one or more of said major surfaces of the melt mixture wherein the composition and thickness of the barrier layer cause it to inhibit gas diffusion such that the time it takes for the fugitive gas to diffuse out of the melt mixture into the atmosphere is substantially greater than the time it takes for the fugitive gas to nucleate and expand, and
4) causing the melt mixture to foam.
Another aspect of the invention provides a method of varying the density of a foam article comprising:
1) varying the amount of fugitive gas in a foamable polymer melt mixture, and
2) during the shaping stage of making a foam article, varying one or both of the thickness and composition of a non-foaming barrier layer applied to or created on at least one major surface of the shaped foamable melt mixture to control the difference between diffusion time and foaming time.
Various aspects of the invention employ carbon dioxide as the fugitive gas. Carbon dioxide may be provided as carbon dioxide in the form of a physical blowing and/or carbon dioxide produced by a chemical blowing agent. Other gases are also suitable for use as fugitive gases, as is explained further in the Detailed Description section.
The methods of the invention are especially effective when high concentrations of fugitive gas are used in combination with an unfoamed barrier layer. The fact that incorporating unfoamed barrier layers on a foam layer causes a reduction in foam density is counterintuitive. Intuitively, one would expect the unfoamed barrier layers to increase the density of a foam article. However, it is believed that the presence of the barrier layers changes the gas diffusion characteristics of the fugitive gas, causing more of the gas to nucleate and grows cells in the polymer matrix as opposed to escaping from the polymer matrix of the melt mixture into the atmosphere.
One aspect of the invention discloses a technique whereby as the thickness of the unfoamed barrier layer increases for a given combination of polymer matrix and fugitive gas concentration and type, the density of the foam structure decreases. In some instances, the decrease is about 200%.
As used in the present application:
xe2x80x9cfugitive gasxe2x80x9d means a gas that has a propensity to diffuse out of a polymer into the atmosphere when exposed to atmospheric pressure, preferably a gas having a vapor pressure of greater than 0.689 MPa at 0xc2x0 C.
An advantage of at least one embodiment of the present invention is that using unfoamed barrier layers to control the diffusional properties of fugitive gas(es) alleviates the need to use flammable hydrocarbons or complicated cross-linking technology to produce high performance foams.
Another advantage of at least one embodiment of the present invention is that it enables low cost production of thermoplastic foams such as polyolefin foams using gases such as CO2 as the sole blowing agent.